Fog has become widespread across Sydney and very thick in some areas early this morning, causing travel headaches. Recent rain has been a major contributor, increasing the available moisture required for fog formation. The ample moisture has combined with light winds and low temperatures to produce fog so thick that visibility has dropped to just a few hundred metres in many suburbs. The light winds have been just strong enough to transport the fog from the western suburbs (where it was initially thickest) towards the city and eastern suburbs, including Sydney Airport in Mascot. Visibility at the airport has dropped below 500 metres at times, low enough to lead to flight delays and even some diversions. Fog is likely to begin burning off soon after sunrise, although some high cloud will slow this process down. This means some fog is still likely to be lingering until after 9am, potentially an hour or so after in the more sheltered areas. Looking ahead to tonight and tomorrow morning, fog is again likely although it is expected to be less widespread and generally not as thick due to wind being a bit stronger.